Neutralized amplifier system



April 27, 1926.

L. C. F. HORLE NEUTRALIZED AMPLIFIER SYSTEM Filed A ril 24, 1925 //V VE N 7'0 1. AW/PE/YCE GE HOALE ATTORNEY Patented Apr. 27, 1926.

. UNITED s'rA T E TSJfPATjEu-Tor Law-Rance c. r. HI tLE, 0F NEWARK, NEW tenses, assre von r0 FEDERAL TELE- 1 rnonn MANUFACTURING Como-Barron, or BUFFALO, NEW.YORK, aconronm TION on NEW yonx.

nnurnhuz nio meme-1E3 SYSTEM. v I I Application filed A rn 24, 1925. Serial in. 25,490.

To all whom it may aonaermf a resident of Newark, I State of New Jersey, have invented certain I I This invention relates to'the Be it known that I, LAWRENCE C. F.- Hon1.n, a citizen of the United States, and county of Essex, and

new and useful Improvements in- Neutralized Amplifier Systems, of which the following is a specification.- T 7 control and suppression of feed-backs, or reaction cur rents, in electrical systems, and particularly to the neutralization, to anydesired; degree, in amplifier circuits-comprising interlinked grid and plate circuits of aitube, of the in-' herent grid plate capacity coupling through- .which the plate circuit reacts on film grid circuit to produce disturbing oscill' tion's.

As is wellknown, the control, or the neutralizing to any desired degree, of ,u'miesirable regeneration-effects in amplified circuits employing the three electrode tube may be accomplished by a large variety of adaptations .of the Wheatstone bridge prin ciple.- In these adaptations, certain circuit elements of the amplifier system constitute A the four arms of the bridge. ,One of the arms comprises the capacity which is to'be 1 wholly or: partially.neutralized, this being the inherent capacity coupling between the grid and plate circuits of a tube. The other arms may also. be substantially capacitive,-

or certain of the arms may be partly ,or wholly made up of inductance elements, the four arms being arranged to operate as a bridge Systemin connection with other elements of the amplifier circuits.

- In one of these many possible combinations of amplifier circuit elements in a Wheatstone bridge arrangement, two of the arms are substantially capacitive and two substantially inductive. Of the two capaci tive arms one comprises the inherent gridplate capacity the other comprises thein-v I ,herent grid filamentcapacity. The said grid filamentcapacity may be supplemented bv a condenc r connected across grid and filament. The inductive arms comprise inductive elements connected across plate and fil-- ament.- y In order to secure effective and substantially uniform neutralizing action over a wide range of frequencies and under varying 'operatin conditions in an. amplifying Lsystem embo ying' the abovemited arrang r must be maintained at or frame elements, connected to earth if desired. The circuit elements in connection with which the matinput and output circuits, oftuning elements if tuned circuits are emment of abridge structure comprisingtwo substantially capacitive and two susbtantially'inductive arms, provision. must be made in the circuit connections to. minimize or annul the disturbances due n general to stray capacltics whichact in such a manner as to 'ntroduce reactions tending todestroy'- or ren er diflicult and uncertain the balance of the bridge. To avoid these disturbances due to stray capacities in various the system, certain elements in said system c 'the samepotential, as by grounding said elements to some com-" .mon convenient mass comprised in the system, such, for example, as a frame element,

which in turn may be ter of grounding, for the purpose turbances assumes first. importance comprise or defining of potentials,

parts ofof avoiding-capacity dismovable members ployed, and current sources such as the cathode battery 4 or' other current supply means and mechanism. I

Of the above-mentioned amplifier ele ments, the cathode current source in any given system is ordinarily and so located that one of its terminals is grounded, as on a convenient mass .such as 'a frame element forming a fixed part of the' preferably system, and such usual grounding of the a cathode currentsource is provided in the,

circuit arrangements embodying the present t invention But inorder to avoid disturbances due to stray capacities above referred to, it is necessary that one side also of both the input and output circuits of a given tube'y -be brought to the-same potential as the frame element or other convenient mass to' which one 't'erminal'of the cathode battery is connected. Such grounding is necessary Whether these circuits are tuned oruntuned.

In untuned circuits the grounding is re-,

quired because the said disturbing effects of stray capacity in and about windings and ther reason that anyftuning element'fcomprised-1n these .c1rcu1ts,for example, a

connections appear when saideircuits are 'left-i'ingrounded; If the circuits are tuned,

ing condenser, necessarily has a movable member, which is manually operated and which, therefore, will be subject to disturbances due to hand capacity it the circuit which it tunes is left ungrounded.

It is required, therefore, that one side of these circuits be grounded to the common grounding mass on which the said current source is also grounded. It is, however, evitube elements, and special means are re- -quired-for the grounding of the said output circuit.

Further, in a brid e system of the type described herein,the 1nput circuit is coupled across a corner intermediate the two capacitive arms and a' corner intermediate the two inductive arms. Therefore in order to permitthe grounding of one. side of the input circuit to the common grounding mass on which the cathode current source is grounded when the said input circuit is conductlvely coupled to the bridge corners, the cathode.

current source must also be connected to the bridge corner intermediate the two inducp two-inductive arm bridge arrangement in an tive arms of the bridge. Means must therefore be provided in that inductive bridge arm which is between the cathode and the corner inter1nediate cathode and anode, whereby. heating current from said current source may through said arm.

For convenience in ex sition, the amplifier'system to which the ridge arrangement is adapted has been stated herein to comprise. a single three electrode thermionic device withits related circuits. It is to be-understood, however, that the invention is direeted not but also to a multi-stage system. The same difliculties involved in the'ad'aptation of the above mentioned bridge to the .single stage amplifier are encountered to the same or eater degree inthe case of a multi-stage, or reasons which will be readily apparent. In order to meet the problems above outlinedthereis provided in accordance with the present invention an improved circuit arrangement for an amplifier system embodyinga Wheatstone bridge structure of the type stated, whereby disturbingcapacity effects may be avoided, said circuit arrangement providing for the grounding, preferably-to the common groundingmass, such -as a frameelement or elements Q1 hich the There-' he conducted to the cathode only to such single stagesystem' cathode current source is grounded, of one side of the lnput circuit or circuits, of the thermionic device, .or devices. Said circuit arrangements further provide for similar grounding of the output circuit or circuits, and in case any or all said input andoutput circuits arc-tuned, for the similar grounding of the movable members of tuning eondensers or other tuning elements.

In general the invention providesfor an effective disposition of the various elements of the system and particularly for the dc fining of the potential of certain elements with reference to a common grounding mass comprised in the system, and a resultant minimizing of disturbing current reactions such as those due to stray capacities, so that an amplifier embodying the well known bridge arrangement herein described may be easily and economically constructed and may be operated with high ,efiiciency over a wide range of frequencies, without disturbing effects due to undesirable cathode circuit to anode circuit reactions.

The circuit arrangement employed in carrying my invention into effect, and its principle of operation, will be clearly understood -by reference to the following description, and the accompanying drawings.

amplifier system. The circuits shown in F igures 1 and 2 are identical, but for clearness the circuits are arranged in bridge form in Fig. 2, and in the. conventional radio circuit diagram form in Fig. 1.

Fig. 3 illustrates an amplifier system embodying the invention and comprising a plurality of tubes.

Referring to Fig. 1, vacuum tube 1 comprises filament 2, grid 3 and plate 4. Grid 3 is connected in an input circuit 5, one side of'which is grounded to a convenient mass such as a frame element 24 and which may be tuned by variable condenser 6 of WlllCh the side 7 will beconsidered the rotor. Plate 4 is connected in a plate-filament circult 8, comprising, in series, coil 9 and in- 'd uctive element 10. Inductive element 10 is composed of the two preferably closely coupled coils 11 and 12. Filament battery' 13 and rheostat 14 are connected to the filament through the said coils 11 and 12. Plate battery 15 is connectedinto the plate filament circuit,'pref erably between coils 9 and.

11. This battery may also be connected between coils 9 and12 ;Conden ser 16 is con nected acrossfilament 2;and grid 3. Condensers- 17 and '18, shown ;i n dotted line,.

represent respectively the: inherent capacity between grid and plate circuits and between grid and filament. The plate filament circuit 8 is inductively coupled to an output circuit 19, one side of which, is grounded l asto a frame element 24 and which coma prises coils and 21, coupled, with a coil 9 and inductive element 10. 19 may be tuned as by a variable condenser 22- of which the side 23 is to be considered the rotor. Rotor 7, of condenser (i, and bat tery 13 are grounded to a convenient mass such as the frame element 24. Likewise rotor 23, of. condenser 22, is grounded as to frame element '24". Radio frequency by- 15 passv condensers may be included, as

shown in the figures, if desired.

In operation, incoming signals from any outside source are'impressed, by way of the input circuit 5, between grid and filament, W and the resultant amplified voltage in the plate circuit is impressed on the output circuit 19. Oscillations due to feed-back from plate-filament circuit 8' to grid-filament cirwit 5, through the electrostatic coupling 25 formed by the inherent capacity 17 between grid-and plate circuits, are controlled or neutralizedby the balanced bridge action incorporated in the system. The bridge action of the circuits is obvious from an in-' spection of Fig. 2. The four arms of the bridge are seen by reference to the said Fig. 2 to comprise; inherent grid-plate circuit capacity 17 (the capacity coupling through which the undesirable feed-back takes place) v between the corners G and P; inherent gridfilament capacity18 together withadditional 'gridfilament capacity,'w'hich-may be added, in the form of condenser 16, between-corners G and F; the impedance comprising 0 "coil"9,'between corners P and X; and the impedance comprising inductiveelement 10 made-up of the two coils 11 and 12, between corners F and Since capacities 17 and 18 are fixed for any given type of tube, the

input of a'second tube 26. The inductively coupled output circuit 19 of tube 1' prefei' ably constitutes the input circuit of this second tube. ,The anode cathode circuit 27 of tube 26 is. arranged in the'same manner 2 as described in. connection with anodecathode circuit 8 of tubel, and the placing of current-sources, cathode, coil w1ndings and grounds is the same as in the case of.

corresponding elements about, the first tube 5 of the series. Circuit 27 may be coupled to The circuit balance of the bridge is-determined by the' the input circuit of a succeeding tube (not shown). Input circuits 5,19, 28 are grounded to a common convenient grounding mass such as a frame element or frame elements of the system, as 29. Likewise one terminal of the cathode battery 13 (Fig. 3) is grounded to such frame element, as at 29; Radio frequency by-pass condensers as 30 are preferably provided in the anode-cathode circuits. The well understood action of the simple bridge arrangement of the type herein described is seen to be in no wise compli-' cated by reason of its adaptation to amplifier circuits, and it is evident that no diflicuh, ties arise inthe proportioning of the ampli fier elements making up there'specti'vearms of the bridge or in securing the desired complete or partial balance of the bridge system. C rtain serious difficulties, however, are encountered, as before suggested, in the adapting of this type of bridge to the vacuum tube amplifier, due to the complexity of structure and the exacting requirements as to the relation of various elements to each other'a-nd to ground in an amplifier system as compared with the usually simple form of the Wheatstone bridge proper. How the complete amplifier system which embodies the present invention is developed in such a manner on the simple two capacitive arm, two inductive arm bridge as to meet these di culties is best explained by referring again to Fig. 2. As before stated, one side of both input and output circuits of a given tube are required to be grounded to a common grounding mass such "as a, frame element of the system, with the movable member of the tuningelement, if the circuit. is tuned, connected to thegrounded side. At'thesame time the said'circuits necessarily must terminateon, or be coupled across, different corners of the bridge. Inorder to meet this difficulty, in the present;

invention I arrange to connect one of the circults-the input circuit 5 termlnating on points G and X.conductive,ly intothe system as usual, and to groundon'e side of this circuit 5,, and if the circuit is tuned, the

movable member 7 of any tuning element such as condenser 6, as at frame element 24. But I arrange the other 'rcuit in such relation to thelsystem that 'it is conductively separated off from the bridge structure of the preceding tube and then grounded on one side, with'the movable member of any turning element connected therewith also grounded. This necessary conductive separating "off of the circuit which is on the out-' put side of a giventube, and which contains a movable element also requiring grounding if the circuit is tuned, is acomplished by the provision ',o f i the extra winding comprised 1n coils 20 and 21, this winding being in,-- ductively coupled with the series coils 9 and 1 0 of. plate circuit 8.. The resulting con-J I a frame element at 24". If the output circuit is tuned, as by condenser 22, the movable member 23 of this tuning. element is also grounded by reason of its connection to the grounded side of the circuit.

It will be noted that to provide this conductive separating off of the circuit 19 on the output side of a given amplifier tube,

advantage is taken'of the fact that in. the two capacitive arm, two inductive arm bridge system herein used for the purpose of controlling or balancing out feed-backs, the two arms between'poin'ts P and F through X, to which points the output oncuit of the tube is coupledfiare the inductive arms of the bridge, in series. Therefore, it becomes possible to utilize these arms com prising coils 9 and 10 in the inductive coupling with the isolated and 'separately grounded tuned circuit 19. In efiect the in. ductive arms 9 and/ 10 of the bridge become the primary of a coupling transformer of which the windings and 21 constitute the secondary.

Further, as before explained another difficulty arises in the adaptation of this simple bridge to the three-electrode vacuum tube amplifiersystem. This difliculty involves the necessary placing of the'ca-thode current source at the corner ofthe bridge intermediate the cathode and anode, this placing being required in order to permit the simultaneous grounding of one side of the conductively coupled input circuit and a terminal, of the cathode battery to the common grounding mass. The problemis met in the present invention by arranging the circuits in such a manner that the cathode as awhole is ata potential different from ground (i. e.,-ott ground) or different from the potential of either terminal of the cathode current source. The cathode is'removed from such ground or current source potential by the inductive element (10) forming or comprised in one of the bridge arms. This elementextends 110111 the cathode corner of the bridge to the corner intermediate the cathode and anode.

The cathode current source, with one -termi-' nal grounded to some common mass such as a frame element, is connectedinto the system at said corner which is intermediate the two inductive arms of the bridge. In order to provide a path from the said current source to the cathode, the said inductive element. is constructed in the form of twocoils, conductively separated but inductively coupled. For this two-winding construction of the inductive element 10, use ispreferably made of the duplex-wound coil which forms part of the subject-matter of mvcopendingapplication Ser. No. 614608, filed January 24, 1923. It is to be understood, however, that said inductive element 10 is not restricted in its construction to the form described in,

said copending application, as other forms of coupled coils may be utilized for the purpose of the present invention.

Filament battery 13, or other current source, is thus not connected directly across filament 2, as this placing would result in separating the battery from grounding mass 24 by one of the inductive arms (10) of the bridge, Utilizing the two-winding element 10, the battery 13 is connected to the filament through the two coils ;11 and 12 which constitute said inductive element. Since one -side of input circuit 5 is connected also to coil 12 and at the end of said coil farthest from the filament (point X of the bridge) and since, further, this side of the input circuit is grounded (as at grounding mass 24), therefore, this method of connecting the in- )ut circuit 5 and the filament battery 13 rings oneend-of said battery to the potential of the grounding mass 24, also to the potential of one side of the input circuit, and too that of the movable member of the tuning element if this circuit is tuned. It is to be noted from the above description that in the circuit arrangement for cathode current source connections herein disclosed, a cathode current, for example, a direct current provided by a battery or a heating-current of any suitable character 'from any source, is carried from the current source to the cathode over conductive elements which in themselves are primarily inductive elements forming a radio frequency inductance common to the cathode-anode circuit and the cathode-control electrode circuit.

It is obvious that the balancing of the bridge system for alternating current is not appreciably affected by the substitution of the two-winding inductive element '10 forv It is to be noted further that by the above described placing of. battery 13, utilizing the two-winding inductive element l0to locate said battery 13 between the two inductance arms ofthe bridge instead of at the filament end of these arms in series, it is possible to place the plate battery 15 adjacent the filament battery .13, as in usual racti ce. In the present embodiment plate attery 15, is conveniently placed between coils 9 and 11.

It will be apparent from the foregoin discussion of the circuit arrangements disc osed in the present invention for the purpose of adaptingthe two capacitive arm, two inductance arms bridge to 'amplifier use, that proper grounding of an input circuit and at the same time of current sources to some common grounding mass such as a frame element of the system is accomplished, in

general terms, by locating the cathode ofi ground, thereby permitting the establishing as a ground point for the bridge of that corner of said .bridge which is between-tho two inductive arms.

As applied to an amplifier including a three-electrode vacuum tube of well known type and operating on the frequencies usual in radio telephony, I find that dimensions for various elements of the system herein described which have given efiicient results in operation, are as stated below. Coil 9 is of nine turns 3 diameter. Double wound coil 10 (made up of the twoooils 11 and 12) is of ,three double turns, 3 diameter. Close paralleling of the urns makes the coupling between coils 11 and 12 substantially unity. Coils 20 and 21 are arranged as a single coil of 54 turns, 3 diameter.- This 54 turn coil, constituting coils 20 and 21- of the figures, is preferably loosely coupled to. coils 9 and'10. Condenser 16 iiused is preferably df a form which permits close adjustment for a final setting of the bridge for the purpose off-neutralizing feedback, the condenser 16 then being locked for this setting. An appropriate capacity range for this condenser 16 is from 20 mmfd. to

35-mmfd. In'describing the circuits 5 and 19, I have shown optional tuning means as. comprising condensers 6 and 22 with manually movable rotors. It is to be understood that these well known tuning meansmay be replaced by any other which will accomplish the purpose of tuning the said circuits 5 and 19. The ,tuning condensers 6 and 22 or other equivalent means, form no partof the present invention and require no detailed description as to arrangement or magnitudes.

In the description of the invention referonce has been made to a three electrode vacuum tube comprisingl grid,;plate and filareceiving systems, particularly for radio frequency amplification. It will be apparout, however, that the invention may bejextended to many other applications. Circuit arrangements in accordance with the invention may be applied, for example, to systems comprising radio-frequency amplifiers .arranged in bridge'form as herein described and also comprisingother. amplifiers not arranged in the said-"bridge, form. Said circuit arrangements may likewise .be applied to audio-frequency amplifiers; or to radio transmitting apparatus;v or to line. 4:. In an electrical system comprising 'a I 'cathode," said electrica wire transmitting and receiving apparatus; orto devices for the prevention of radiation from the antennas of radio receiving systems. It is evident that these and still fur-' 'ther applications of the invention other than those of the embodiments shown may be made, and that various modifications in the circuit arrangements and apparatus employed may also be made, without depart- ,mg from the scope of the invention as set fort-11in the appended claims.

What is claimed is:

1. In an electrical system comprising a thermionic device having a control electrode, an anode and a cathode, aninput circuit including the control elect-rode andthe cathode,

a circuit including the anode and the cathode,

'frame' element at a point intermediate said inductive bridge arms.

2. In an el e ctrical system comprising a thermionic device having a control electrode,

an anode and a cathode, an input circuit including the control electrode and the oathode, a circuit including the anode and the cathode, a frame element for said system, a

source being grounded to the frame element, said electrical system being arranged as a bridge for the controlling f feed-back action from the anode-oath current source for the cathode, said current e circuit to the input circuit, said bridge comprising an inductive arm between said cathode and a bridge corner intermediate saidcathode and anode, said bridge beinggrounded to said frame element at said corner, and means comprisin said inductive arm for maintaining sa' (1 cathode at a potential different from that of said frame element.

3. In an electrical system comprising a an anode and a cathode, an input circuit inode,-a circuit-iiicludin theanode and the thermionic device having a control electrode, v

system being arranged as a bridge for the controlling of feed-back action from the anode-cathode circuit to the input circuit, said bridge comdiate said cathode and anode, a current source for the cathode, and means comprising said inductive 'arm for conducting heating ourrent from said current source to said cathode and simultaneously forconducting radio frecircuit.

quency oscillations in said anode-cathode tween said current source and thermionic device having a control electrode, an anode and a cathode, an input circuit including the. control electrode and the cathode, a circuit including the anode and the cathode, said electrical system being arranged as a bridge tor the control of feedback action from the anode-cathode circuit to the input circuit, said bridge comprising an inductive arm included in said anode- I0 cathode circuit and having two inductively coupled windings, one of said windings being included in said input circuit, a current source for the cathode, and means includin said windings for bringing one side of said input circuit-and a terminal of the said current source to the samepotential and to a ,i potential substantially different from that of the cathode. V

5. In an electrical system comprising a thermionic device having a control electrode,

an anode and a cathode, an input circuit including the control electrode and the oathode, a circuit including the anode and the cathode, a frame element for the system, saidsystem being arranged as abridge for the control of feed-back action vfrom the anode-cathode circuit to the input circuit, the anode-cathode circuit comprising two inductivebridge arms, one of said two arms havin twoinductively coupled windings, one oi saidwindings being included in said input circuit, tuning means for the input ,circuit comprising aituning element having an adjustable member, a current source for 85 the cathode, a circuit inductively coupled to the anode-cathode circuit and having one side grounded to saidframe element and comprising a tuning element having its. ad

justable member grounded to said frame element, and means including said windings for bringing one side of the'said input circuit and the said adjustable member of the input circuit tuning element and one terminal of the said current source to the potential of said frame element and to a potential substantially different from that of the cathode.- v

6. In an amplifier, a three-electrode ther- .mionic device comprising an anode, acath- 60 ode, andacontrol electrode, input and anodecathode circuits for said amplifier, said am- .plifier being arranged in balanced bridge form for controlling feed-back action from the anode-cathode circuit to the input circuit, one oflthe arms of said bridge being inductive and included in said anode-cathodecircuit and having two inductively coupled windings, a current source for the cathode, and means comprising said inductlve arm for bringing one side ofsaid input circuit "and one terminal of said current source to the same .-potential, one of said windings being connected directly in said input cir-' cuit, both said windings bein connected besaid cathode.

the anode-cathode circuit comprising two in 7. In an amplifier, a three-electrode thermionic device comprising an anode, a cathode and a control electrode, input and anode-cathode circuits'i'or said amplifier, a frame element for said amplifier, said amplifier being arranged in balanced bridge form for controlling feed-back action from the anode-cathode circuit to the input circuit, twof'of the arms of said bridge being inductive'and included in said anode-cathode "5 circuit, one of said two armsbeing located between the cathode and an adjacent corner of the bridge and having two inductively coupled windings, a current source for the cathode,--a circuit inductively coupled to the anode-cathode circuit, one side of said inductively cougpled circuit being grounded to said frame element, and means com rising said\windin'gs for bringing one si e of said input circuit and one terminal 'of said current sourceto the potential of said frame element, one of said windings being connected directly in said input circuit,.both said windings being connected between said current source and said cathode 90 8. In an electrical system comprising a thermionic device having a control'electrode, an anode and a cathode, a tuned input circuit including the control electrode and the cathode, a circuit including the anode and 95 the cathode, said electrical system being arranged as a bridge for the control of feedback action from the anode-cathode circuit to the input circuit, said bridge comprising an inductive arm having two inductively coupled windings, one of said windings being included in said input circuit, tuning means for the input circuit comprising a tuning element having an adjustable member, a current source for the cathode, and

means com risin said windin s for brin ZD D ing the said adjustable member and one terminal of the said current source to the same potential-and to a potential substantially different from-that of the cathode.

9, In an'electrical system comprising a thermionic device having a control electrode, an anode and a cathode, a tuned input circuitincluding the control electrode and the cathode, a circuit including the anode and the cathode, a frame elem ent for the system, said system being arranged'as a bridge for the control of feed-back action from the anode-cathode circuit to the input circuit,

120 ductive bridge arms, one of said two arms being connected to the cathode and extending to an adjacent bridge corner intermediate cathode and anode and having two inductively coupled windings, tuning means for the input circuit comprising a tuning element having an adjustable member, a current source for the cathode, a tuned circuit inductivel coupled to the anode'cathode circuit an havingone side grounded to said no ment having=.its adjustable member ground ed tosaid frame element, and means comprising saidg-windings, for br nging one side of the said input circuit and the said adjustable member of the said input circuit tuning element and one terminal of the said current source to the potential of said frame element and to a potential substantially dif--\ ferent from that of the cathode.

-10. In an amplifier, a three-electrode thermionic device comprising an anode, a cathode, and a control electrode, input and anodecathode circuits for said amplifier, said amplifier being arranged in balanced bridge form for controlling feed-back action from the anode-cathode circuitto the input circuit, one of the arms of said bridge being inductive and included in said anode-cathode circuit and having two inductively coupledwindings, tuning means for said input circuit comprising a tuning element havmg an adjustable member, a current source for the cathode, and means comprising said inductive arm for bringing said adjustable member and one terminal of said current source to the same potential, one of said windings being connected directly in said input circuit, both said windings being connected between said current source and said cathode.

' 11. In an amplifier, a three-electrode ther mionic device comprising an anode, a cath-.

ode and a control electrode, input and anode-cathode circuits for said amplifier,

said amplifier being arranged in balanced bridge form for controlling feed-back action from the anode-cathode circuit to the input circuit, two of the arms of said bridge being inductive and included'in said anodecathode circuit, one of said two arms being located between the cathode and an adjacent corner of the bridge and having two inductively coupled windings, tuning .means for said input circuit comprising a tuning element having an adjustable'member, a current source for the cathode, a tuned circuit inductively coupled to theanode-cathode circuit and comprisinga tuning elementhaving an adjustable member at ground potential, and means comprising said windings for bringing said adjustable member of the.

said input circuit tuning element and one terminal of said current source tov ground potential one'of said windings being connected dlrectly in said input circuit, both said windings being connected between said current source and said cathode.

In witness whereof, I have hereunto signed my name.

LAWRENCE C. F. HORLE. 

